An example of an optical fiber unit used in an optical submarine cable or the like is shown in FIG. 1. FIG. 1 is a cross section, wherein 1 indicates a center tensile wire, 2 indicates multiple optical fiber core wires twisted together around the center tensile wire 1, 3 indicates an inner layer unit solid material filled between the optical fiber core wires, to protect them, and 4 indicates an outer layer unit solid material. 5 indicates an optical fiber unit thus constituted. Here, the outer layer unit solid material 4 is made of a relatively hard resin, to protect the optical fiber core wires from external force, and the inner layer unit solid material 3 is made of a relatively soft UV resin, to protect the optical fiber core wires 2 by alleviating external force. Generally, since photocurable resins have the advantage that they cure quickly and are good in productivity, they are used as resins for solid materials of optical fiber units. This optical fiber unit 5 is used, for example, as a core of an optical submarine cable, as shown in FIG. 3.
FIG. 3 is a cross section, wherein 11 indicates an optical fiber unit (the same as shown in FIG. 1), which is held in a composite metal body composed of an iron three-section pipe 12, a tensile piano wire 13, and a copper tube 14. A waterproofing resin 15 is filled between the optical fiber unit 11 and the iron three-section pipe 12, and between the tensile piano wires 13. 16 indicates an insulating layer, and 17 indicates a corrosion-preventive layer (jacket). 18 indicates an optical submarine cable thus constituted. The above metal composite body also functions as a feeder for repeaters or the like.
In the meantime, in the production of this optical fiber unit, while multiple optical fiber core wires 2 are being twisted together around a center tensile wire 1, a photocurable resin is filled into the spaces between the optical fiber core wires, to cover the optical fibers. At that time there arises the problem that tiny bubbles inevitably remain in the covering layer of the solid material made of the cured resin on the optical fiber core wire side since the twisting is carried out with involvement of bubbles in the photocurable resin. It is known that if a number of such bubbles are present around the optical fiber core wires, it causes an increase in transmission loss due to microbends.
Accordingly, such bubbles have to be prevented from remaining.
Further, in addition to the prevention of bubbles from occurring, this type of optical fiber unit is required to have basic properties: It is required that, when the terminal is processed to take out the optical fiber core wires to be connected to the optical fiber core wires of another optical fiber unit, the optical fiber core wires can be easily removed from respective solid materials constituting the optical fiber unit; that is, the workability must be excellent; and the force required for withdrawing the center tensile wire from the photocurable resin of the solid material must not be decreased.
As a conventionally proposed optical fiber unit, one is described in JP-A-61-14210, wherein multiple optical fiber core wires, each circumference of which is coated with a covering layer, are gathered around a center member and are covered with., as a cushioning medium, an ultraviolet-curable resin composition containing a specified acrylic oligomer and a triacrylate of tris(2-hydroxyethyl)isocyanurate. While this, however, improves the heat and humidity resistance, the elastic coefficient, and the like of the cushioning medium, it does not take the problem of the occurrence of microbends due to the involvement of bubbles into consideration at all.
Further, JP-A-62-47008 describes that multiple optical fibers, each covered with an ultraviolet-curable resin, are directly twisted together, an ultraviolet-curable silicone resin is filled into spaces between a center tensile wire and the multiple optical fibers and spaces between the said multiple optical fibers, and two solid layers of an ultraviolet-curable resin, different in Young's modulus are formed around them, but the aim here is to improve the heat resistance, thereby preventing transmission loss from increasing, and to increase the basic required property, i.e. the workability to make terminal working easy.
Therefore, an object of the present invention is to provide an optical fiber core wire binder comprising a, resin composition for covering an optical fiber that forms a cured product (unit solid material) that involves less bubbles, that has fewer remaining bubbles, and that is excellent in adhesion to a center tensile wire (tension member) and in workability when it is filled and applied around optical fiber core wires while they are being twisted.
Further, another object of the present invention is to provide an optical fiber unit wherein bubbles at the interface between optical fiber core wires and a solid material made of a resin-cured product around them are kept from remaining, and the adhesion between the solid material and a center tensile wire, and the workability, are excellent.
Other and further objects, features, and advantages of the invention will appear more fully from the following description, taken in connection with the accompanying drawings.